1. Field
This invention relates generally to low voltage detection circuits and more specifically to a brown-out detection circuit disposed on an integrated circuit and fabricated using a complementary metal oxide semiconductor (CMOS) process.
2. Related Art
Some data processing or other electronic systems include a low power, or stand-by, mode where power is removed from most of the system during certain times to reduce power consumption. It is often desirable to maintain a power supply voltage to a small portion of the system, such as a static random access memory (SRAM), to retain certain critical memory contents. However, an SRAM requires a minimum data retention voltage to prevent data loss. During low power mode, the power supply voltage provided to the SRAM may become too low for reliable data retention. Also, a drop in the power supply voltage can occur anytime and can be due to a variety of reasons. When the supply voltage to the SRAM falls below the minimum data retention voltage, it is important to inform the system of the possibility of corrupted data.
A brown-out is a condition of low supply voltage (VDD) in a system that adversely affects system operation. A brown-out may be caused by a variety of ordinary glitches, including inadequate power regulation, powering up or down system components, and software bugs. Portable electronic devices, such as cellular phones and cameras, are particularly susceptible to brown-outs because a connection between the device's battery and its terminals may be interrupted by even slight movements.
During a brown-out, if VDD drops below a required voltage of the system, data may be corrupted and the system may malfunction. Because the condition of circuit elements is lost during the brown-out, the system must then be reset to resume operation. Therefore, accurate brown-out detection is vital to ensuring reliable operation in most systems.
In general, a brown-out detection circuit is used to monitor the power supply voltage and to provide a warning when the monitored voltage falls below a predetermined voltage. The system can then take corrective action. More specifically, a brown-out detection circuit is used to detect when VDD falls below the minimum data retention voltage. A brown-out detection circuit is used to prevent data corruption and system malfunction. When VDD falls below the minimum data retention voltage, a brown-out detection circuit generates a system reset signal to set logic within the system to a known state. Therefore, when VDD returns to a stable operating potential, the system may resume operation from the reset state.
Known brown-out detection circuits are relatively inaccurate, so a relatively high voltage supply is required to assure correct system operation. As improving process technologies allow for the use of progressively lower power supply voltages, the accuracy of the brown-out detection circuit becomes more important.
The system typically has an unregulated voltage that varies between 0.9 to 5.0 volts, and the unregulated voltage is regulated to 1.2V±10%. When the unregulated voltage drops below 1.2V, the regulated voltage, VDD, also drops below 1.2V. When VDD drops below a certain voltage (typically a voltage between 1.08V and 0.95V, if VDD is 1.2V), most of the circuits of the system should stop, and the system goes into standby mode. When a system is in stand-by mode, a brown-out detection circuit needs to continue to work, and needs to continue to monitor the voltage of the power supply for the SRAM. While in standby mode, no reads or writes can take place, but the SRAM retains data. When VDD drops below about 700 mV, the entire system, including the SRAM, should totally stop because the SRAM can not reliably retain data when VDD is below about 700 mV.
SRAM may perform reads and writes at an operating voltage of about 1.2V. An SRAM is not able to retain its contents at voltages below its brown-out voltage. For an SRAM that has an operating voltage of about 1.2V, the minimum data retention voltage can be as high as 700 mV. At or below the highest minimum data retention voltage, the contents of the SRAM may be compromised.
Most brown-out detection circuits monitor only the threshold of one type of transistor (for example, either NMOS or PMOS type). In such known brown-out detection circuits, the variation (such as with temperature or process) of the brown-out detection voltage and the variation (such as with temperature or process) of the minimum data retention voltage may not track each other accurately. Therefore, a higher than desirable threshold detection range is required to assure correct system operation. The threshold detection range is critical for smaller geometries due to the lower supply voltage.
The performance of known brown-out detection circuits varies with process and temperature. Some known brown-out detection circuits disadvantageously use polysilicon resistors, which vary with temperature and process differently than how NMOS and PMOS devices vary with temperature and process.
Many brown-out detection circuits are not area-efficient, and some brown-out detection circuits are power hungry and/or too complex for System-on-Chip (“SoC”) applications. Some brown-out detection circuits employ open-base bipolar transistors that are seldom available in a standard CMOS process.